1. Field of the Invention
The present invention relates generally to lightweight proppants for oil and gas wells and more particularly to lightweight proppants having a specific gravity of about 1.3 or less.
2. Description of the State of Art
After a well has been drilled to the projected depth and the production formations have been evaluated as economical to produce, the work of setting the casing, preparing the well for production, and bringing in the oil or gas begins. In general, many oil and gas wells 10, shown in FIG. 1, require four concentric strings of large pipe: conductor pipe 12, surface casing 14, intermediate casing 16 and production casing 18. Conductor pipe 12 is cemented C in place and prevents the hole from caving in at the surface and endangering the drilling rig foundation, not shown. Surface casing 14 is set and cemented in place to provide protection for fresh water formation. Surface casing 14 also prevents loose shell and sand or gravel from falling into the hole or wellbore B and affords a means for controlling the flow of fluid from the well 10. Intermediate casing 16 may be needed if troublesome zones are encountered below the surface casing 14 and above the final depth of the well 10. The final casing for most wells is the production casing 18 which is also set in cement.
Once the process of setting the production casing 18 in place has been accomplished the well next undergoes completion. The type of completion method used is determined by the characteristics of the reservoir and its economical potential. The perforated completion, which is by far the most popular method of completing a well, is accomplished by piercing the production casing wall 20 and the cement C thereby providing openings through which formation fluids may enter the wellbore B. Piercing the casing 18 is accomplished by lowering a perforating gun (not shown) down the production casing 18 until it is opposite the zone to be produced. The gun is fired to set off special explosive charges known as shaped charges that are designed so that an intense, direction explosion is formed resulting in perforations 22.
Since oil and gas usually exist in the pores of the formation surrounding the wellbore B, enlarging or creating new channels causes the oil or gas to move more readily to a well 10. Hydraulic fracturing, shown in FIG. 2, is a well stimulation process of injecting fluids (not shown) into a selected oil or gas bearing subsurface earth formation traversed by a wellbore B at sufficiently high rates and pressures such that the formation fails in tension and fractures to accept the fluid. In order to hold the fracture F open once the fracturing pressure is released a propping agent P or proppant is mixed with the fluid which is injected into the formation. Hydraulic fracturing is used to accomplish three tasks: (1) create penetrating reservoir fractures to improve the productivity of a well, (2) improve the ultimate recovery from a well by extending the flow channels further into the formation, and (3) aid in improved recovery operations.
To achieve the maximum width of a propped fracture upon release of the fracturing pressure and thus the maximum flow of fluids from an oil or gas reservoir to a wellbore B the transport and placement of proppant are of major importance. Both transport and placement of proppant rely on the ability of the fluid to carry the proppant into the fracture. If the specific gravity of proppant to the specific gravity of fluid ratio is greater than about 2 to 1 the proppant will not be carried or transported into the fracture with the fluid; but instead, will fall out of the fluid commonly referred to as "screen-outs" and fill up the bottom of the well. Therefore, as the specific gravity of the proppant increases a higher viscosity fracturing fluid is required to transport the proppant. See "Factors Affecting Gravel Placement in Long Deviated Intervals", SPE 19400:7-20, Formation Damage Control Symposium, Lafayette, La., February, 1990.
In general, proppants are strong particles that are capable of withstanding the high temperatures and pressures associated with a fracture. Early proppants were formed of materials such as sand, glass beads, walnut shells, and aluminum pellets. However, where closure pressures of the fracture exceed a few thousand pounds per square inch these materials are crushed resulting in a closure of the fracture. In response, proppants having high compressive strength have been designed to resist crushing under high pressure levels experienced in use. While these proppants prove to have sufficient strength to resist crushing they also have high specific gravities of about 2.0 or more requiring the use of higher viscosity fracturing fluids. The conductivity or crushability of a proppant under specific conditions of stress, temperature, corrosive environment and time is the single most important measure of its quality.
It is well recognized that a side-effect of all fracturing operations with high viscosity or gelled fluid is the potential for formation damage from filtrate invasion. R. Puri, et al. in their scientific paper entitled "Damage to Coal Permeability during Hydraulic Fracturing," SPE 21813:109-115, Proc. Rocky Mountain Regional and Low-Permeability Reservoirs Symposium, Denver, April, 1991, disclosed that higher viscosity fracturing fluids cause damage that is irreversible to coal permeability. Coal consists of a highly cross-linked macromolecular network and other uncross-linked macromolecular chains. Therefore, coal has a high capacity to sorb a wide variety of liquids and gases. Field studies reported by Puri et al. demonstrate that 20-30% of the injected frac gel volume is not recovered, and is suspected to be trapped in the coal. Furthermore, "it appears that even water containing low concentrations of friction reducing polymers can cause significant damage to coal permeability. These results are alarming since extensive damage to permeability by gelled fluids and friction reducing polymers could negate most of the benefits of a large effective wellbore radius created during an expensive fracture stimulation . . . . It is recommended that every effort be made to avoid contacting coals with gelled fluids, polymers, or liquid chemicals. Furthermore, remedial workover treatment should be considered for coal wells that could have been damaged in the past by hydraulic fracture stimulation." Id. at 109.
There is still a need, therefore, for a proppant having a specific gravity of 1.3 or lower that maintains sufficient compressive strength to resist fragmentation under high stress levels thus alleviating the need to use viscous fluids.